As it happens I own a DT880 600 ohm. I use the CEntrance DACport as a portable DAC+Amp combo and it drives it very well. You could take a look at Schiit Valhalla. You really don't need to roll tubes to get good sound out of it. On the other hand, you could also look for a CEntrance DACmini CX which has the same headphone amplifier as with the DACport only with a much better power supply. The DAC in the DACmini is much talked about for it's prowess. It is well beyond the $500 budget but look for one used or replace your existing DAC with the DACmini, if it's not a step back that is ! But I'm quite sure you'll be happy with the way it drives the DT880.